1. Field of the Invention
This invention relates to polymeric materials. More particularly, it relates to a polymeric material which is crosslinked at low pH with conventional gelatin crosslinking agents. The crosslinked polymers of the present invention can be coated on various supports to provide useful elements, particularly photographic elements.
2. Description of the Prior Art
It is well known that certain units can be included in a copolymer to provide for the later crosslinking of the polymer. Typically, conventional units which provide for the crosslinking of the polymer have had activated methylene groups, amine groups or hydroxyl groups. Unfortunately, known units of these types do not provide for crosslinking at low pH. Because of this deficiency, a large number of copolymers cannot be crosslinked using conventional crosslinking units. For example, polymers of acid comonomers containing conventional crosslinking groups must be neutralized before they can be crosslinked.
Frequently, it is desirable to disperse various components which are sensitive to pH in crosslinkable polymers. Thus, in addition to the fact that the polymer itself might hydrolyze at high pH (about 5 or higher), a component that is dispersed in the polymer might also be affected if the polymer dispersion was crosslinked at high pH. For example, photographic silver halide is usually coated in a polymeric binder and is optimally coated in the range of pH 4.5 to 6.5. The higher pH that is sometimes necessary to initiate crosslinking with conventional crosslinking units can adversely affect the silver halide. In addition to silver halide, other pH sensitive components can be coated in polymeric binders to form, for example, useful integral bio-analytical elements. Typical integral analytical elements are described, for example, in U.S. Pat. No. 3,992,158 to Pryzbylowicz and Millikan and the reference cited therein.
Typically, the active methylene group of a crosslinkable polymer is located in a side chain and is linked to the main chain of the polymer through an ester group. In one patent relating to active methylene group-containing polymers, namely, U.S. Pat. No. 3,904,418, crosslinked polymers which are particularly resistant to hydrolysis are described. The patent does discuss the disadvantages of hydrolysis of crosslinked polymers at high pH; however, since the crosslinking reaction itself must be carried out at pH values above about 5, hydrolysis of other components included with the polymer can occur.
One area where conventional crosslinking polymers are extremely useful is as extenders for gelatin in photographic elements. Due to its unique properties, including its ability to disperse silver halide and its excellent protective colloid properties, gelatin has been used as a binding agent in layers of photographic elements for many years. Gelatin is, however, vulnerable to dimensional change when subjected to varying conditions, such as temperature and humidity. Gelatin can also hydrolyze to amino acid fragments when subjected to extremely acidic or basic conditions. Crosslinking polymers have been proposed as substitutes or partial substitutes for gelatin in one or more layers of photographic elements to improve dimensional stability. As mentioned above, however, conventional crosslinking polymers must be crosslinked with care when used with silver halide emulsions, due to the fact that the silver halide is sensitive to pH. Some silver halide emulsions cannot be directly used with these conventional crosslinking polymers. This complicates, and therefore makes more expensive, the photographic film manufacturing process.
It is readily apparent that there is a continuing need for crosslinkable polymers which can be crosslinked with conventional gelatin hardening agents at relatively low pH.